The “WiMAX” communication standards for mobile stations primarily stipulate, as base station switching procedures (hereafter “handover”), two types, “Controlled HO” and “Uncontrolled HO” (see for example IEEE 802.16e-2005).
Controlled HO is a handover procedure such that, when a mobile station (MS: Mobile Station) moves, the base station (BS: Base Station) is switched after negotiation with the currently connected base station (BS).
That is, when a mobile station (MS) decides on a neighbor base station (NBS: Neighbor BS) as a handover destination, the mobile station (MS) receives periodically (for example, in 30 second intervals) neighbor BS information (MOB_NBR-ADV) from the currently connected base station (SBS: Serving BS), and measures the radio wave strength of the neighbor base station (NBS) based on identification information (BSID) of the neighbor base station (NBS) contained in the neighbor BS information. Then, based on the radio wave strength and service level (SLP: Service Level Prediction) of the neighbor base station (NBS), the mobile station (MS) finally decides the neighbor base station (NBS) to be the handover destination. The service level Prediction is information indicating whether service flow can be handed over, that is, whether the current service flow can be handed over to the destination base station (BS), and is a parameter obtained from the serving base station (SBS) in the HO-REQ/RSP sequence. FIG. 12 shows an example of service level prediction (SLP) information.
On the other hand, uncontrolled HO is an emergency recovery procedure for a case in which communication of a mobile station (MS) with the serving base station (SBS) is interrupted prior to completion of a controlled HO procedure; a connection is made to a base station (BS) with which connection is possible based on radio wave strength, without performing an HO sequence with the serving base station (SBS). In uncontrolled HO, the mobile station (MS) does not execute the HO-REQ/RSP sequence, and so service level information cannot be utilized.
For example, as shown in FIG. 11, a case is considered in which a mobile station (MS) 100 is handed over to a certain base station (BS#1) 200-1, and has left the communication range (cell) of this base station (BS#1) 200-1 in approximately 30 seconds.
FIG. 13 shows the sequence example for this operation. The base station (BS#1) 200-1 broadcasts neighbor BS information (MOB_NBR-ADV) (S101). However, the mobile station (MS) 100 is not connected to the base station (BS#1) 200-1, and so cannot receive the neighbor BS information.
Thereafter, the mobile station (MS) 100 moves to within the cell range of the base station (BS#1) 200-1, and executes the connection procedure (S102). The mobile station (MS) 100 attempts to perform handover when the radio wave strength of the base station (BS#1) 200-1 reduces, but is not receiving the connection BS information, and so has no identification information for a neighbor base station (NBS), and cannot execute the HO-REQ/REP sequence (S103). The mobile station (MS) 100 finds a neighbor base station (BS#2) 200-2 based on radio wave strength, and performs uncontrolled HO (S104, S106 to S109).
When the mobile station (MS) 100 receives information (“SLP=0”) indicating that taking over service level is not possible from the target base station (BS#2) 200-2 (s109), the mobile station (MS) 100 executes a sequence to reregister (reset) a new service flow with the target base station (BS#2) (S110 to S112).
However, even if the uncontrolled HO procedure is executed, the target base station (BS#2) 200-2 cannot necessarily takes over the service flow of the serving base station (BS#1) 200-1. On the other hand, the mobile station (MS) 100 cannot confirm whether service flow can be taken over until after handover (S106 to S109). Upon obtaining the information that the service flow cannot be taken over (SLP=0), the mobile station (MS) 100 performs re-registration of the service flow (S110 to S112), so that much time is required for handover.
Even when the mobile station (MS) 100 uses the controlled HO procedure executing the HO-REQ/RES sequence with the serving base station (BS#1) 200-1 (S103), and receives service level information, it performs service flow re-registration, if taking over service level at the target base station (BS#2) is not possible (SLP=0).
In this way, in the case of uncontrolled HO or in the case that service flow is not taken over under controlled HO, the mobile station (MS) 100 performs service flow re-registration if service flow cannot be handed over between the handover destination base station (BS#2) and the serving base station (BS#1). As a result, there is the problem that time is required for handover.